Windows won't even begin to load for 5 minutes

Whenever I boot my computer (Dell Dimension 8300, 8 years old, with Windows XP Professional, SP3), I get the Dell splash screen, indicating that the BIOS is loading, etc.

Then it turns into a black screen, with a cursor line blinking in the upper left corner. It does not even appear that Windows is trying or starting to load.

After about 5 minutes of this, Windows finally begins to load, loads in a reasonable amount of time, and then works perfectly normally and at normal speeds, until the next time it is re-booted.

I have had the motherboard tested, showing normal. I have replaced the hard drive with a new faster larger one, with no change (I did NOT reload Windows, just restored everything to my drive using an Image Restore).

It was working fine until a couple of months ago, then starting acting as described above.

I can live with the time to load Windows, etc., but am concerned that this could be a symptom of something about to fully die, and would rather know about it ahead of time, in case I have to make some repairs/replacements while I still can while it is still functional.

Does the described behavior occur with all booting or just cold bootups?What has changed around the time this started happening?

You could start by looking through the event viewer for errors relating to your problem. It's possible there might be something there.
I would suggest clearing it out first if it has not been done in a long time. Then allow the computer to go through it's normal paces
of shutting down and booting before looking.
I would also start by trying the usual things; Perform a full AV/AM scan, cleaning and defragmenting, & check out Process Explorer and Autoruns (Sysinternals) for
any potential clues.

Well, one thing you can bet on..... windows is doing something! It's not just taking a coffee break.

My own PC boots up real fast, compared to some other PC's I have to work on, but...... if I have my Epson 6000 AIO printer turned on at the time I try to boot up, the motherboard sees those USB ports on the printer and will try to boot from them first. My PC may eventually boot up, but only after a long wait. So when it doesn't boot up quickly, I start looking for things like my printer being ON, or some other USB card installed, etc.
Once the problem is removed, I hit the reset button on the front of my PC and then it boots up normally.
No reset button? Then hold your power switch ON for ten seconds to shut down the PC, then try again to boot normally.

Obviously something is wrong. All you need to do is find what it is, eliminate it and your PC will boot up normally.

Sounds to me that the BIOS is having a hard time validating hardware or is attempting to boot first from a non bootable device. As DR Who suggested, unplug all USB cables to make sure none of them are causing the boot delay.Try going into BIOS and making sure all your hard disk and cd/dvd drives show up. Change the boot priority to boot from your Window disc first. (you may have to change it back to CD/DVD first when you need to boot from them if the BIOS doesn't support a separate function key for CD/DVD boot) Open the PC and make sure all the drive cables are secure.

I agree. It sounds suspiciously like the BIOS s having problems completing. Have you got the BIOS screen displaying on Start Up or do you get a Dell splash screen? Try going into BIOS and see if you can turn off the Splash screen so that you can monitor the progress of the BIOS starting. With an 8 year old system, it could be anything from dirty connections to a failing PSU motherboard or even Hard drive.